This invention relates to the field of semiconductor processing, and more particularly to a method for forming an etch mask and exemplary uses therefor.
During the manufacture of a semiconductor device a large number of transistors and other structures are formed over a semiconductor substrate assembly such as a semiconductor wafer. As manufacturing techniques improve and transistor density increases as feature size decreases, one manufacturing step which can create difficulties is photolithography, as there is a limit to the minimum feature size which can be formed with conventional equipment.
Various attempts have been made to overcome the limitations of conventional photolithography. For example, U.S. Pat. No. 5,750,441 by Figura et al., assigned to Micron Technology, Inc. and incorporated herein by reference in its entirety, describes various patterning techniques which have been developed in an attempt to decrease the allowable feature size using conventional lithographic equipment.
Using an oxygen plasma treatment to alter the etch characteristics of a material has been demonstrated. Hicks, et al. (S. E. Hicks, S. K. Murad, I. Sturrock, and C. D. W. Wilkinson, xe2x80x9cImproving the Resistance of PECVD Silicon Nitride to Dry Etching Using an Oxygen Plasma,xe2x80x9d Microelectronic Engineering, 35, pp. 41-44, 1997) teaches the treatment of a silicon nitride layer to increase its resistance to an etch. After the silicon nitride layer is formed using plasma enhanced chemical vapor deposition, it is subjected to a treatment in a reactive ion etch chamber comprising a radio frequency power of 50 Watts, an oxygen flow rate of 25 standard cubic centimeters, and a gas pressure of 100 millitorr thereby resulting in a direct current bias of 110 volts. Hicks teaches a layer which is homogeneously densified. The etch rate of an untreated silicon nitride layer using SF6 reactive ion etching was demonstrated to be up to 1,000 times greater than a treated silicon nitride subjected to the same etch conditions.
A patterning technique which can form device features smaller than those allowable by conventional photolithography equipment would be desirable.
The present invention provides a new method which decreases the minimum device feature size that can be formed with conventional photolithography equipment. In accordance with one embodiment of the invention a semiconductor substrate assembly is provided and a layer of polycrystalline silicon (poly) is formed thereover. A silicon nitride layer is formed over the poly layer, and a silicon dioxide layer is formed over the silicon nitride. The nitride and oxide layers are patterned with an etch mask having a width, thereby resulting in cross sectional sidewalls in the silicon nitride. The sidewalls of the silicon nitride are treated with an oxygen plasma which alters the etch characteristics of the silicon nitride sidewalls.
The silicon dioxide is removed, as is the untreated portion of the silicon nitride layer thereby resulting in pillars of treated silicon nitride having a width less than the width of the mask. The poly layer is etched using the silicon nitride pillars as an etch mask, thereby resulting in poly features, each of which has a width less than the width of the original etch mask.